1. Field of the Invention
The invention relates to a adjustable resistance device comprising a first parallel arrangement of a first resistor and a first positive-feedback transconductor, which parallel arrangement has a control input coupled to an output of a control circuit.
Such an adjustable resistance device is suitable for general purposes and is particularly suitable for realizing resistance values which are comparatively insusceptible to expansion of the resistance tolerance range.
2. Description of the Related Art
The parallel arrangement of a resistor with a positive-feedback transconductor is known, inter alia from U.S. Pat. No. 5,220,686, where it is included in the resistance-determining section of a resonance amplifier. By constructing the resistance-determining section as a parallel arrangement of a first resistor and a first positive-feedback transconductor, the effective resistance can be increased since the first positive-feedback transconductor functions as a negative resistance arranged in parallel with the first resistor. By selecting the value of this negative resistance to be a factor k (k&gt;1) larger than the value of the first resistor the effective resistance of the parallel arrangement of the first resistor and the first positive-feedback transconductor increases by a factor M=(k/k-1). This has the advantage of a higher effective resistance without a large chip area or a higher supply voltage being required. In the resonance amplifier the transconductance of the transconductor, and hence the effective resistance value in the parallel arrangement, is controlled by means of a control circuit.
However, the resistance increase by means of a parallel-connected negative resistance is subject to the restriction that in the case of an increase of the effective resistance by a factor M, a deviation in the positive or negative resistance results in a deviation in the effective resistance which is M times as large. For very high effective resistance values, which are attended with a very high value for M, a comparatively small relative tolerance range of the positive or the negative resistance value will result in a substantial tolerance range in the effective resistance value. This means that an external adjustment of the effective resistance value is necessary.